In the engineering machinery industry manufacturing truck cranes, concrete pump trucks, concrete mixer trucks and the like, an increasing number of enterprises gradually increase the proportion of high-strength structural steel that is used. In design of new vehicles, a strategy of “increased strength and decreased thickness” is employed, and upgrading and updating of products is accelerated at the same time. Up to now, high-strength steel having a yield strength at the levels of 600 MPa and 700 MPa has been used widely. The use of high-strength steel having a yield strength of 800 MPa or higher is quite limited. In the compositional design of hot-rolled high-strength steel of Grades 600 MPa and 700 MPa, a high amount of titanium is added in most cases for predominant precipitation strengthening, and the structure is mainly granular bainite. High-titanium high-strength steel comprising granular bainite structure generally has a ductile-brittle transition temperature of about −40° C., and the impact performance varies greatly. At the same time, a use environment of −30° C. to −40° C. is required by some engineering machinery users, and a higher strength is also required. Under such a background, high-titanium hot-rolled high-strength steel not only fails to satisfy the requirement of strength, but it's more difficult for this steel to ensure low-temperature impact toughness. Hence, it's urgently necessary to develop a high-strength, high-toughness steel material having low cost.
Low-carbon or ultralow-carbon martensite is a multi-sized structure. The strength of low-carbon or ultralow-carbon martensite mainly depends on the size of lath bundles, and there is a Hall-Petch relationship between the strength and the size of lath bundles. As the size of lath bundles decreases, the strength and toughness of steel increases. Fine martensitic lath bundles can prevent propagation of cracks more effectively, so as to promote the low-temperature impact toughness of low-carbon or ultralow-carbon martensite steel. It's just on the basis of this concept of designing ultralow-carbon martensite that the present invention is proposed.
Chinese Patent Application No. 03110973.X discloses an ultralow-carbon bainite steel and a method of manufacturing the same. Because the end cooling temperature after water cooling is between the bainite transformation temperature Bs and the martensite transformation temperature Ms, or in a range of 0-150° C. lower than Bs, the strength of the steel is rather low. Even if relatively high amounts of Cu and Ni are added and medium- or high-temperature tempering is performed, the maximum yield strength of the steel plate is still lower than 800 MPa, and the structure is mainly ultralow-carbon bainite. In addition, when the Cu content exceeds 0.4%, tempering treatment must be conducted, which increases process steps and manufacture cost. Hence, the method disclosed by this patent application can only produce a series of high-strength steel having a relatively low strength, and the yield strength cannot reach 800 MPa or higher.
Chinese Patent Application No. 201210195411.1 discloses an ultralow-carbon bainite steel and a method of manufacturing the same. The essential design concept of this patent application is still the use of ultralow-carbon bainite with relatively precious alloy elements such as Cu, Ni, Cr, Mo and the like added in amounts as less as possible. Instead, addition of a medium amount of Mn is employed in the design concept. That is, the Mn content is controlled at 3.0-4.5%. It's well known that, when the Mn content is 3% or higher, the mechanical properties of the steel plate can be desirable. Nevertheless, for a steel plant, such a high Mn content will cause extreme difficulties in steel making, particularly in continuous casting, because cracks tend to be generated in a steel blank during continuous casting, and fracturing can easily occur during hot rolling, resulting in poor utility. Moreover, the carbon content in Example 4 is up to 0.07%. This amount of carbon is no longer ultralow carbon in its general sense.